1. Field of the Invention
The present invention relates to a liquid crystal display device.
2. Discussion of the Related Art
Until recently, display devices have typically used cathode-ray tubes (CRTs). Presently, many efforts and studies are being made to develop various types of flat panel displays, such as liquid crystal display (LCD) devices, plasma display panels (PDPs), field emission displays, and electro-luminescence displays (ELDs), as a substitute for CRTs. Of these flat panel displays, LCD devices have many advantages, such as high resolution, light weight, thin profile, compact size, and low voltage power supply requirements.
In general, an LCD device includes two substrates that are spaced apart and face each other with a liquid crystal material interposed between the two substrates. The two substrates include electrodes that face each other such that voltages applied to the electrodes induce an electric field across the liquid crystal material. Alignment of the liquid crystal molecules in the liquid crystal material changes in accordance with the intensity of the induced electric field into the direction of the induced electric field, thereby changing the light transmissivity of the LCD device. Thus, the LCD device displays images by varying the intensity of the induced electric field.
FIG. 1 is a cross-sectional view illustrating an LCD device according to the related art.
Referring to FIG. 1, the LCD device 1 includes a liquid crystal panel 30 and a backlight unit 90 below the liquid crystal panel 30. The liquid crystal panel 30 includes first and second substrates 5 and 10, a liquid crystal layer 15 between the first and second substrates 5 and 10.
Although not shown in the drawings, a gate line and a data line crossing each other and defining a pixel region P are formed on the first substrate 5. In the pixel region P, a switching region S is defined where a thin film transistor Tr is formed.
The thin film transistor Tr includes a gate electrode 25, a gate insulating layer 45, a semiconductor layer 40, and source and drain electrodes 32 and 34. The gate electrode 25 is connected to the gate line, and the gate insulating layer 45 covers the gate electrode 25. The semiconductor layer 40 is on the gate insulating layer 45 and corresponds to the gate electrode 25. The semiconductor layer 40 includes an active layer made of intrinsic amorphous silicon, and an ohmic contact layer made of extrinsic amorphous silicon. The source electrode 32 is connected to the data line, and the drain electrode 34 is spaced apart from the source electrode 32. The source and drain electrodes 32 and 34 are on the semiconductor layer 40.
A passivation layer 55 is formed on the thin film transistor Tr and includes a drain contact hole DCH exposing the drain electrode 34. A pixel electrode 70 is formed on the passivation layer 55 of each pixel region P. The pixel electrode 70 is made of a transparent conductive material.
A black matrix 12 is formed on the second substrate 10 and shields the gate and data lines and the thin film transistor Tr. A color filter layer 16 is formed on the black matrix 12 and includes red (R), green (G) and blue (B) color filter patterns 16a, 16b and 16c in the respective pixel regions P. A common electrode 75 is formed on the color filter layer 16 and is made of a transparent conductive material.
Although not shown in the drawings, first and second alignment layers are formed on the pixel and common electrodes 70 and 75, respectively.
The backlight unit 90 supplies light to the liquid crystal panel 30. A cold cathode fluorescent lamp (CCFL), an external electrode fluorescent lamp (EEFL), a hot cathode fluorescent lamp (HCFL), a light emitting diode (LED) or the like may be used for the backlight unit 90.
Recently, it is requested that an LCD device is seen in a specific color when the LCD device is in display off state. The LCD device is generally seen in gray, for example, black when the LCD device is in display off state. However, recently, electronic devices, for example, mobile stations employing the LCD device and a colored exterior case are produced. Accordingly, when the LCD device is in display off state, a user may feel that the black of the LCD device is not in harmony with the colored exterior case of the mobile station. To satisfy such the need, an LCD device using reflection property of cholesteric liquid crystal is proposed.
FIG. 2 is a cross-sectional view illustrating an LCD device using reflection property of cholesteric liquid crystal according to the related art.
Referring to FIG. 2, the LCD device 50 includes a liquid crystal panel 30, a backlight unit 90 below the liquid crystal panel 30, and a cholesteric liquid crystal film 80 on the liquid crystal panel 30.
The liquid crystal panel 30 has a structure similar to that of the liquid crystal panel of FIG. 1. In other words, as shown in FIG. 1, the liquid crystal panel 30 may include the first substrate 5, the second substrate 10 and the liquid crystal layer 15 between the first and second substrates 5 and 10.
In display on state, the backlight unit 90 emits light to the liquid crystal panel 30, thus the liquid crystal panel 30 produces images. The produced images through the liquid crystal panel 30 passes through the cholesteric liquid crystal film 80, thus the LCD device 50 finally displays the images viewed by a viewer.
The cholesteric liquid crystal film 80 includes third and fourth substrates 82 and 84, a separator 88 between adjacent pixel regions P, and a cholesteric liquid crystal layer 86 filling a space defined by the third and fourth substrates 82 and 84 and the separator 88. The cholesteric liquid crystal layer 86 includes cholesteric liquid crystal molecules 87.
The cholesteric liquid crystal molecules 87, for example, functions to transmit right-handed circularly polarized light and reflect left-handed circularly polarized light. Accordingly, the LCD device 50 can be seen in a specific color in display off state since the cholesteric liquid crystal film 80 selectively reflects an external light.
The cholesteric liquid crystal molecules 87 are arranged in helical twist along a vertical direction. The specific color seen due to the reflection of the cholesteric liquid crystal film 80 is determined by a helical pitch and a thickness of the cholesteric liquid crystal layer 86.
However, since the LCD device 50 requires the cholesteric liquid crystal film 80 additionally, manufacturing processes are complicated and production cost increases.
Further, a desired color is seen at front viewing angles, but other color is seen at side viewing angles.
Further, polarizing films are located on top and bottom surfaces of the liquid crystal panel 30. Among lights passing through the polarizing film on the top surface of the liquid crystal panel 30, left-handed circularly polarized light passes through the cholesteric liquid crystal film 80. Accordingly, light transmissivity of the LCD device 50 is reduced.